The present invention relates to a sequence controller, and more particularly to a programmable controller (hereinafter called "PC") for sequence control which is programmable by a user.
Conventionally, programs for a sequence control PC have been made by using either a conditional type definition typified by a Relay Ladder chart or a sequential type definition typified by a step incremental method. The control to be performed, however, often contains both a part suitable for conditional type control and a part suitable for sequential type control. In view of this, a PC operating on the basis of a combination of both these definitions has been proposed as shown in JP-A-58-66114.
According to JP-A-58-66114, the whole specification for sequential control is configured by using two types of conditional and sequential definitions, and the programs with the two definitions are executable by a single PC. According to this technique, programs using the conditional type definition are grouped into a plurality of conditional control blocks to thereby distinguish them from a plurality of sequential control blocks into which the other programs using the sequential type definition are grouped. With this conceptual technique, the conditional and sequential control blocks are formed, therefore, without any relation to the physical structure of an object to be controlled. For this reason, it is difficult to understand the physical structure of a controlled object from the sequence control specification, and conversely to form a sequence control specification properly matching the physical structure of the controlled object. There is accordingly some difficulty in making programs.
Similar problems also arise even if a Petrinet graph presently attempted in the art is used.
If a mechanism is to be sequentially controlled by a single PC, a plurality of connection wirings L are extended from the single PC to various physical elements, such as switches, motors and the like as shown in FIG. 8. As a result, not only do the number of wirings become enormous, but also the structure becomes complicated so that many accidents become possible. In addition, the scale of programs to be executed by the PC becomes large, resulting in a difficulty in making and maintaining programs.